[SLP][NFC]Fix compile build by declaring ArrayRef, NFC.

Fix compiler build reported in https://lab.llvm.org/buildbot#builders/243/builds/218

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -4083,7 +4083,8 @@ void BoUpSLP::reorderNodeWithReuses(TreeEntry &TE, ArrayRef<int> Mask) const {
   // Clear reorder since it is going to be applied to the new mask.
   TE.ReorderIndices.clear();
   // Try to improve gathered nodes with clustered reuses, if possible.
-  SmallVector<unsigned> NewOrder(ArrayRef(NewMask).slice(0, Sz));
+  ArrayRef<int> Slice = ArrayRef(NewMask).slice(0, Sz);
+  SmallVector<unsigned> NewOrder(Slice.begin(), Slice.end());
   inversePermutation(NewOrder, NewMask);
   reorderScalars(TE.Scalars, NewMask);
   // Fill the reuses mask with the identity submasks.